1. Field of the Invention
The present invention concerns a system for scanning along an arciform trajectory with a variable bending radius.
It can be applied especially, but not exclusively, to the echography of ocular structures, it being understood that it can also be used for guiding miscellaneous tools or instruments, both in the medical field and for controlling and machining spherical or aspheric materials having a convex or concave geometry.
2. Description of the Prior Art
In ultrasonic imagery and more particularly in medical echography, the fineness of details (spatial resolution), the contrast of the image and the accuracy of the measurements depends on the frequency and focal distance of the ultrasonic probe, as well as the performances (geometry, extent, accuracy, speed) of the system for scanning the ultrasonic beam. The choice concerning the frequency and the focal distance of the probe depends on making a compromise concerning resolution and the penetration depth. In fact, owing to the increase of the attenuation of the ultrasonic waves with the frequency, the depth of penetration of the ultrasounds is much greater when the frequency is low. On the other hand, the resolution of the images reduces.
The accuracy of the measurements made on the image depend on the resolution, but also on the orientation of the ultrasonic beam with respect to the structure to be explored. In addition, the precision and reliability of an echographic examination are more important when the volume of the explored issue is large. Thus is the reason a 3D scanning adapted to the geometry of the explored organ allows a volume imaging of the structures, an improved definition of their contours and an accurate localisation of lesions.
In ophthalmology, the 2D echography at 10 MHz is used in current practice for exploring the anatomy and pathologies of ocular structures and more particularly those of the posterior pole (retina, optic nerve, vitreous table). The technique uses a sectorial scanning making it possible to obtain cutaway images (2D) with spatial resolutions close to one millimetre. Although sectorial scanning is not adapted to the bend of the cornea, an extremely rough examination of the entire anterior segment (cornea, iris, anterior chamber, crystalline lens) can however be obtained at 10 MHz. The use of higher frequencies, typically between 50 and 80 MHz, allows fine imaging of these structures.
Several systems functioning at 50 MHz (spatial resolution of 50 μm) have been highly developed.
The first, UBM (Ultrasound Backscatter Microscope) is a 2D echograph abler to explore in real time (eight images/s) with a linear scanning the structures of the anterior segment. The range of scanning is limited to a zone measuring 5 mm long and 5 mm deep. This limitation is due to the fact that the ultrasonic beam is only perpendicular to the central portion of the cornea. So as to produce an image of the entire cornea, it is necessary for the scanning to follow a curvilinear (or arciform) trajectory whose bending radius is close to the average bending radius of the cornea.
The patent application WO 01/49181 describes a system which carries out a scanning along a circular trajectory whose radius can be adjusted so as to approximately correspond to that of the eye of the patient.
This system includes two parallel rocker bars joined by one of their extremities to a structure for supporting the probe and by the other extremity to a rotating plate of a drive mechanism, the unit comprising the rocker bars, support structure and plate constituting an articulated parallelogram. These two rocker bars are joined by their central portion on a second plate identical to the first and coupled to the latter so as to rotate in synchronism with the latter. This rotation causes a translation movement of the rocker bars which remain parallel to an axis passing through the rotation centre of the two plates. The probe carries out a circular trajectory centered on said axis. During this rotation, the probe remains orientated towards the centre of the circular trajectory. An adjustment of the diameter of this trajectory can be obtained by varying by means of two cams the distance between the centres of rotation of the plates and the hinge pins of the rocker bars on these plates.
The drawback of this solution is that it only makes it possible to carry out circular trajectories of the probe and does not take account of the specific shape of the cornea.
Now the cornea is not really spherical and has large variations between its centre and the periphery: the further one is away from the centre of the cornea, the more the bending radius of the latter increases.
In fact, as shown on the accompanying FIGS. 1 and 2, the basic plane of the cornea 1 (eye seen from top) has an elliptical shape with a large diameter D of about 12 mm (perpendicular to the axis of the nose) and a small diameter d of about 11 mm (parallel to the axis of the nose), the difference of diameter originating from the opening and closing of the eyelids.
Moreover, it is a known fact that the cornea 1 has two zones, namely a central zone which is spherical (which corresponds approximately to the pupil area) and a peripheral zone in which the bending radius progressively increases towards the limbus. Thus, it appears that the cornea 1 is an aspheric and asymmetrical calvaria which progressively flattens towards its periphery. The average radius of the anterior face is about 7.8 mm and that of its posterior face is about 6.7 mm. The thickness at the centre of the cornea is about 0.5 mm and 1.2 mm at its periphery at the level of the limbus. Owing to the various bending radii between the cornea and the sclera 2, the joining point of the cornea 1 and the sclera 2 has a visible sulcus 3 (discontinuity) at the level of the irido-cornean angle.
In the rest of this text, the bending radius of the anterior face of the cornea shall be denoted by “the bending radius of the cornea”.
By using a small keratometer, it has been observed that normal cornea exhibit flattening of 3 diopters or more at a distance of 2 to 3 mm from the cornean apex (region with the larger bend) which corresponds to a bending radius of 8.4 mm.
It has also been shown that the normal corneas are aspherical, more bent inward at the centre and exhibiting a progressive flattening towards their periphery. This flattening has an amplitude of 5 to 7 diopters, namely respectively a bending radius ranging from 8.8 to 9.5 mm which represents an increase varying from 12.8 to 21.7% with respect to the average bending radius of 7.8 mm (FIG. 2).
In particular, the further one is away from the centre of the cornea, the more the bending radius of the latter increases. The advantage of arciform scanning is to enable the probe to follow a trajectory whose bending radius is fixed and approximately equal to the average bending radius of the cornea, thus making it possible to have the axis of the ultrasonic beam orthogonal to a major portion of the surface of the cornea. However, this scanning shows several limitations at the periphery of the cornea where the ultrasonic beam no is longer strictly perpendicular to the cornea on account of the variation of the bending radius of the latter. In addition, even if a safety distance (for example 2 mm) is taken with respect to the anterior face of the cornea, because of the bending radius of the trajectory of the front face of the probe is [7.8+2=9.8 mm. As this bend is] smaller than that of the cornea of its periphery (which moves from 9 mm to 12 mm at the joining point with the sclera), the ultrasonic probe would tend to draw close to the sclera with a risk of contact (risk much bigger when the diameter of the probe is large).